[server] extract "no pooler" CnxSet class to a _BaseCnxSet class So we get rid of "if self._queue is None" in each method of _CnxSetPool Also add helper get_cnxset(source, size) to instantiate the correct pooler class.

# re-read ComputedRelation permissions from schema.py now that we're
# able to serialize them
for computedrtype in schema.iter_computed_relations():
    sync_schema_props_perms(computedrtype.type)